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Nanotech discovery could have radical implications

Posted January 3, 2006; 08:44 a.m.

by Teresa Riordan

It has been 20 years since futurist Eric Drexler daringly predicted
a world where miniaturized robots would build things one molecule at a
time. The world of nanotechnology is beginning to come to pass, with
scientists conjuring new applications daily.

Now Princeton scientist Salvatore Torquato
is proposing to turn a central concept of nanotechnology on its head.
If the theory bears out -- and it is in its infancy -- it could have
radical implications for the computer and telecommunications
industries.

Torquato and colleagues published a paper in the Nov. 25 issue of
Physical Review Letters, the leading physics journal, outlining a
mathematical approach that would enable them to produce desired
configurations of nanoparticles by manipulating the manner in which the
particles interact with one another.

This may not mean much to the man on the street, but to the average scientist it is a fairly astounding proposition.

“In a sense this would allow you to play God, because the method
creates on the computer new types of particles whose interactions are
tuned precisely so as to yield a desired structure,” said Pablo
Debenedetti, a professor of chemical engineering at Princeton.

The standard approach in nanotechnology is to come up with new
chemical structures through trial and error, by letting constituent
parts react with one other as they do in nature and then seeing whether
the result is useful.

Nanotechnologists rely on something called “self-assembly,” which
refers to the fact that molecular building blocks do not have to be put
together in some kind of miniaturized factory-like fashion. Instead,
under the right conditions, they will spontaneously arrange themselves
into larger, carefully organized structures.

As the researchers point out in their paper, biology offers many
extraordinary examples of self-assembly, including the formation of the
DNA double helix.

But Torquato and his colleagues, visiting research collaborator
Frank Stillinger and physics graduate student Mikael Rechtsman, have
taken an approach not seen in nature, which they call “inverse
statistical mechanics.”

Instead of employing the traditional trial-and-error method of
self-assembly that is used by nanotechnologists and which is found in
nature, Torquato and his colleagues start with an exact blueprint of
the nanostructure they want to build.

“If one thinks of a nanomaterial as a house, our approach enables a
scientist to act as architect, contractor and day laborer all wrapped
up in one,” Torquato said.

“We design the components of the house, such as the 2-by-4s and
cement blocks, so that they will interact with each other in such a way
that when you throw them together randomly they self-assemble into the
desired house,” he said.

To do the same thing using current techniques, by contrast, a
scientist would have to conduct endless experiments to come up with the
same house. And in the end that researcher may not end up with a house
at all but rather -- metaphorically speaking -- with a garage or a
horse stable or a grain silo.

Paul Chaikin, a physicist at New York University and a former
Princeton professor, said the Torquato paper “presents a first major
success in the solution to an inverse problem.”

“It follows in the tradition of ‘The way to see if you really
understand how something works is to build it from scratch,’” Chaikin
said. “Or even more fundamentally, the new approach shows how to
self-assemble it from scratch.”

While Torquato is a theorist rather than a practitioner, his ideas
may have implications for nanostructures used in a range of
applications in sensors, electronics and aerospace engineering.

“This is a wonderful example of how asking deep theoretical
questions can lead to important practical applications,” said
Debenedetti.

So far Torquato and his colleagues have demonstrated their concept only theoretically, with computer modeling.

They illustrated their technique by considering thin films of
particles. If one thinks of the particles as pennies scattered upon a
table, the pennies, when laterally compressed, would normally
self-assemble into a pattern called a triangular lattice.

But by optimizing the interactions of the “pennies,” or particles,
Torquato made them self-assemble into an entirely different pattern
known as a honeycomb lattice (called that because it very much
resembles a honeycomb).

Why is this important? The honeycomb lattice is the two-dimensional
analog to the three-dimensional diamond lattice -- the creation of
which is somewhat of a holy grail in nanotechnology.

Diamonds found in nature self assemble from carbon atoms that
undergo a type of “directional bonding” that is hard to achieve in
laboratory experiments. The researchers created their pattern with
“non-directional bonding,” which was not previously thought to be
possible. This advance should give experimentalists much more
flexibility in creating useful structures, Torquato said.

Materials with diamond lattice structures are used in high-speed optical communications devices.

To create the honeycomb lattice, the researchers employed techniques
of optimization, a field that has burgeoned since World War II and
which is essentially the science of inventing mathematical methods to
make things run efficiently.

Torquato and his colleagues hope that their efforts will be
replicated in the laboratory using particles called colloids, which
have unique properties that make them ideal candidates to test the
theory. Chaikin said he is planning to do laboratory experiments based
on the work.

“Our colloid group is actively pursuing different types of
interparticle interactions using electrostatics, polymers, DNA
association, van der Waals attraction and entropy which may be combined
to form the types of [interactions] envisioned in this work,” said
Chaikin. “An important aspect of this paper is the simplicity and
robustness built into the types of interactions proposed.”

Torquato said that he and Stillinger initially had trouble
attracting research money to support their idea. Colleagues “thought it
was so far out in left field in terms of whether we could do what we
were claiming that it was difficult to get funding for it,” he said.
The work was ultimately funded by the Office of Basic Energy Sciences
at the U.S. Department of Energy.

“The honeycomb lattice is a simple example but it illustrates the
power of our approach,” Torquato said. “We envision assembling even
more useful and unusual structures in the future.”